1. Field of the Invention
The present invention relates to a magneto-resistive effect type of recording head in which magnetic recording is carried using a magneto-resistive effect element that extends perpendicularly to the magnetic recording medium.
2. Description of the Prior Art
Recently particular attention has been focused on the perpendicular magnetic recording method as a new development in high density magnetic recording techniques. FIG. 1 illustrates the known perpendicular recording method.
Generally designated by 1 is a magnetic recording medium which is shown, in FIG. 1, to be a magnetic tape. The surface of the magnetic recording medium 1 is coated with a layer of magnetic material 2 which exhibits a strong magnetic anisotropy in the vertical direction. The magnetic material layer 2 is 1 to 2 .mu.m thick and is formed, for instance, by spattering of Co/Cr alloy. The arrows indicated by reference character y.sub.1 illustrate the direction of magnetization.
The magnetic material layer 2 is backed up by a back-up layer 4 having a thickness of 1 to 2 .mu.m and being formed using a permalloy of 80% Ni. The presence of this high permeability back-up layer 4 remarkedly improves the efficiency of recording and reproducing. Compared with the magnetic recording medium without such back-up layer, the magnetic recording medium 1 with the back-up layer 4 has 4 to 10 times higher recording and reproducing effect. Designated by 5 is a base film formed of a high molecular synthetic resin such as polyimide.
A main pole 6 and a subsidiary pole 7 are disposed facing each other with the multilayer-structured magnetic recording medium 1, described above passing therebetween. The main pole 6 is referred to also as first pole and forms together with the subsidiary pole 7 a recording magnetic head disposed perpendicularly to the recording medium. The main pole 6 is formed by vapor depositing 80% Ni permalloy (high permeability material) on a substrate. Its thickness tm is on the order of 1 to 2 .mu.m. The subsidiary pole is referred to also as second pole and has a record reproducing winding 8 turned about it. As the material for the subsidiary pole 7 there is used a high permeability material such as MnZn ferrite or permalloy. Its thickness tc is in the order of 100 to 1000 .mu.m and it is so formed as not to be saturated by current flowing into the winding 8. The number of turns of the winding 8 is variable depending on the frequency then used and is suitably selected within the range of from 100 to 1000 turns.
In the above described manner, the known perpendicular magnetic recording and reproducing system uses a magnetic recording medium having three layer structure 1 comprising as one layer magnetic material which exhibits a strong magnetic anisotropy in the perpendicular direction and as another layer a material of high permeability. The magnetic recording medium 1 passes through between the main pole 6 and the subsidiary pole 7. With this perpendicular magnetic recording and reproducing system it is possible to record and reproduce magnetic record wavelengths .lambda. up to 0.3 .mu.m. FIG. 2 is a curve showing the state of reproduction of recorded information obtained with the magnetic recording system described above. Wavelength is plotted on the abscissa and the curve shows the state of digital record with two bits being one wavelength. As seen from the curve, no drop in reproducing output is observed even on the long wavelength side. This is compared with the case of the common induction type magnetic head wherein the reproducing output drops down on the long wavelength side.
In the perpendicular recording system illustrated in FIG. 1, recording/reproducing is carried out through the winding 8 on the subsidiary pole 7. In this case, until now, it has been undesirable to reduce the thickness tm of the main pole 6 to an extremely small value because it causes a reduction of the recording and reproducing efficiency. This limitation has brought about a problem in reproducing the recorded information. The problem is a gap loss caused by the thickness tm. Referring to FIG. 2, it is seen that there appear in the reproducing output curve some dips G.sub.1 -G.sub.5 at the wavelengths of 1.6, 0.8, 0.53, 0.4, 0.32 and 0.27 .mu.m respectively although the recorded wavelength .lambda. extends to 0.3 .mu.m. If the thickness tm of the main pole 6 is further decreased in an attempt at the elimination of the above problem, then the reproducing output may be also decreased to such extent that difficulty arises in processing the signal. The reason for this is that the degree of magnetic flux focussing in reproduction is proportional to the thickness tm of the main pole 6.
In summary, the known perpendicular recording system has such a significant disadvantage that it is impossible to obtain reproducing signals in a stable manner up to the shortest wavelength of the recorded range of wavelength and the reproducing output signal obtainable is very low in signal level.
On the other hand, with the progress of the magnetic recording and reproducing technique in recent years, many attempts are now being made to develop thin film magnetic heads for further improvement of frequency characteristics. Among them, efforts are directed to development of each a recording head which can satisfy the increasing desire for lower running speed of the recording medium and multichannel recording and reproducing. To satisfy the requirement, the number of head elements is increasing and also the degree of integration per head element becomes higher and higher.
For example, speaking of reproducing heads, the conventional induction type thin film magnetic head has a reproducing output voltage proportional to the relative speed of the recording medium relative to the head. Therefore, the reproducing output decreases with decreasing running speed. To obtain a sufficiently large reproducing output even for a low running speed, the number of turns of the coil should be increased to 100 to 1000 turns. With such a large number of turns, the magnetic head is practically difficult to use as a thin film reproducing head.
As a solution to the above problem, there has been developed the magnetic flux response type magneto-resistive effect thin film magnetic head (hereinafter referred to simply as MR head) the output of which has no connection with the relative speed between the head and the recording medium.
An example of known MR head is shown in FIGS. 3 and 4.
Designated at 11 is an electrically insulating and non-magnetic substrate. 12 are Mr Head elements having a magneto-resistive effect. MR head elements 12 are formed on one surface of the substrate 11 on which a magnetic recording medium slide moves. These elements 12 can be formed employing the known thin film depositing technique or other suitable technique. 13 is an electroconductive layer connected with the both ends of MR head element 12. These electroconductive layers 13 are also formed on the substrate 11.
A constant current is applied to MR head elements 12 through the electroconductive layer 13. Under the condition, when an external magnetic field such as a magnetic tape approaches the elements 12, then the resistive value of the elements 12 change. The reproducing output voltage .DELTA.V obtained at this time is represented by: EQU .DELTA.V=.DELTA.RI (1)
wherein, .DELTA.R is change of resistive of the MR element caused by the external magnetic field and I is the value of the constant current applied to the MR element.
The changing rate of resistive change of MR elements 12 caused by an external magnetic field is generally in the order of 1 to 3% for MR element formed of Ni-Fe or Ni-Co. As seen from the aboe equation (1), therefore, in order to obtain a larger reproducing output it is necessary to increase the absolute resistive of each an MR element. The reluctance R of MR element is generally represented by: ##EQU1## wherein, .rho..sub.MR is specific resistive of the MR element;
l is length of the MR element; PA1 w is width of the same; and PA1 t is thickness of the same.
As will be readily understood from the above equation (2), the requirements for a larger resistive of an MR element are to increase the length of the MR element 12 and to reduce the width and thickness thereof.
However, since at present the elements are formed by employing the thin film forming technique, it is impossible to reduce the thickness of the element so much. As for the width of the element, techiques available therefor such as photo etching also have some technical limitations. Therefore, it is impossible to reduce the width to a great extent.
The length of an MR head element 12 corresponds to the track width of the reproducing head. As previously mentioned, at present there is an increasing desire for a high density magnetic recording and reproducing system which needs a multichannel and highly integrated magnetic head. To meet the desire, it is required to reduce the track width, that is, to shorten the length of MR element. As seen from the above equation (2), this means a smaller reproducing output.